KT88 vs 6550 tubes

Yep... you got a bad tube.  I've got a number of BAD tubes from ebay sellers stating them as new.  (I suspect the tubes are factory seconds.) The tubes from certain ebay sellers measure poorer than the ARC supplied tubes with 2000+ hours on them.  I only now purchase from ARC, Tube Depot or Upscale audio.  I've had multiple EH 6992 tubes blow after 30 or 40 hours, from a seller on ebay; I inserted the original ARC Sovtek 6992's (with 1000's of hours on them 2 years ago.) and everything is great.  

   So the “ KT” means kinked tetrode , this was a design change to circumvent the original patent. I have NOS Tung Sol 6550 ( 3 hole ) and SED Winged C 6550’s . I have NOS GEC KT-88’s and KT-77’s (smoked glass ). New production GL KT88’s and KT-77’s. Also new production Psvane, JJ , Tung Sol . Over the last 2 years , I’ve had 1 SED 6550 go bang ( bottle split ) with no amp damage . And one almost new current production GL KT-77 go bad . The latter looked fine but the amp produced a huge pop through my speaker and a wisp of smoke from my amp . It’s not a MAC , but it is “ Self Bias “ . I sent it in for repair . I was told by Mr. Had , that when he tested the tube it was bad and had caused a run away plate voltage issue that toasted the self bias circuit . He also told me on a previous conversation that if I wanted to run NOS 6550’s in my amp to get the “ 3 Hole “ version and not the solid plate version. I’ve run most of the tubes in my Dynakits too . FWIW I like the SED Winged C 6550’s in the Dynakits and the NEW Tungsol KT 120’s in the Had. For late night quiet listening I run either of the Nos GEC ‘s and a 5U4G to drop the voltage . If you don’t want to pay crazy prices for NOS , and want a honest seller ( don’t we all ), try Johnny at 3M Audio in Houston . I use him and I live in California. BTW he’s a high volume Mac dealer . Check his web site . He also lists on Agon and EBay . Another source of info is Chip ( RumDog ) on EBay . I bought a box of SED’s from him . He runs Twin MAC 275’s bi amped with SS on vintage JBL’s ( the big stuff) he has a lifetime of colllecting JBL and is into MAC’s . Cheers , Mike B. 

I’m sorry 3MA Audio , Houston, TX 

I’m a fan of the current production Gold Lion KT-88, just make sure you buy from a reputable seller. I like using McShane. 
Good luck!